In general, a pneumatic tire has a structure constituted of a frame, namely, a toroidal carcass layer extending from one of a pair of bead portions to the other, and surrounding belt layers or belt-reinforcement layers that reinforce the ground contact area and are obtained by rubberizing several kinds of reinforcements.
In particular, conventional carcass ply cords are mainly composed of polyethylene terephthalate (PET) as a polyester fiber or rayon as a cellulose fiber. Known means for decreasing the number of plies while maintaining the strength of the case include the use of para-aromatic polyamide (aramid) fiber to form the ply cords.
Usually, each of the belt layers has a laminated structure constituted of treated material (belt ply) layers each composed of a rubber coated fabric obtained by arranging reinforcing cords in parallel, and the reinforcing cords are made of aramid fiber so that the resulting belts have a reduced weight. However, belt layers reinforced by aramid cords have low rigidity. It is thus customary that at least one of the belt layers, the number of which is usually two or more, is constituted of a belt ply formed using aramid cords and that at least one of the remaining belt layers is constituted of a treated material formed using steel cords.
The belt layers of radial tires for passenger cars are mainly constituted of at least two angled steel belts and sometimes contain a cap layer or layering layers that are rubber coated fabrics formed using reinforcing cords such as nylon cords and arranged substantially in parallel with the circumference of the tire. These layers are intended to ensure driving stability, in particular, to improve durability through prevention of detachment of the belt layers (serious detachment seen especially near the edges of the belt layers) that occurs during high-speed driving. Known means for preventing deformation of the ground contact area during high-speed driving include the use of fiber that has high rigidity and maintains the elasticity thereof even when heated, such as aramid fiber, as a material of the cap layer or layering layers.
Examples of a technique for improving a tire belt include one disclosed in Patent Document 1: a pneumatic tire is realized that has a reduced weight, improved durability, and improved wear resistance, in which at least one of treated materials constituting a tire belt is a rubber coated fabric obtained by arranging aramid cords in parallel so that they are at an oblique angle with respect to the equator of the tire; at least one of the remaining treated materials is a rubber coated fabric obtained by arranging steel monofilaments in parallel so that they are at an oblique angle with respect to the equator of the tire and that they cross the aramid cords; and the interval of the steel monofilaments used in parallel in the treated materials is in the range of 0.10 to 0.50 mm.
Examples of a technique for improving a cap layer or layering layers include one disclosed in Patent Document 2: a pneumatic tire is realized that is well-balanced in terms of road noise, rolling resistance, and flat spot performance, in which the cap layer is divided along the width direction into a middle part and two end parts; the middle part is reinforced by stranded nylon cords having a predetermined twist and a predetermined fineness and settled with a lower embedding number so that the tensile modulus in the direction of the tire circumference per unit width is equal to or lower than 17 GPa/cm; and each of the end parts has an elongation modulus in the direction of the tire circumference of 35 GPa/cm or higher and a width in the range of 8 to 24 mm.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-166812 (Claims and other sections)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-224074 (Claims and other sections)